A selective catalytic reduction-type catalyst (hereunder, also referred to as “SCR”) is known as a catalyst that can purify NOx contained in exhaust gas. An SCR has a function that adsorbs ammonia (NH3), and can selectively reduce NOx contained in exhaust gas by means of NH3.
In Japanese Patent Laid-Open No. 2014-001682, a configuration is disclosed in which a NOx storage-reduction catalyst (hereunder, also referred to as “NSR”) is disposed upstream of an SCR. The NSR stores NOx contained in exhaust gas under a lean atmosphere, and when a reducing agent such as HC or CO is supplied from upstream, the NSR releases the stored NOx to cause the NOx to react with the reducing agent to thereby reduce the NOx to NH3 and N2. Hence, according to this configuration, the NH3 generated when NOx is reduced at the NSR can be adsorbed by the SCR. Further, by utilizing the NH3, NOx that passes by the NSR or NOx that oozes out from the NSR can be selectively reduced at the SCR. That is, according to this configuration, special means such as urea addition means for causing the SCR to adsorb NH3 is not required.
Further, Japanese Patent Laid-Open No. 2014-001682, discloses that, when switching from stoichiometric operation in which operation is performed with an in-cylinder air-fuel ratio that is set at a theoretical air-fuel ratio to lean operation in which operation is performed with an in-cylinder air-fuel ratio that is set to an air-fuel ratio that is leaner in fuel than the theoretical air-fuel ratio, a so-called “rich spike” is executed that enriches for a short period of time the air-fuel ratio of exhaust gas that flows into the NSR. Although the amount of NH3 adsorbed by the SCR decreases during stoichiometric operation, the NH3 adsorption amount of the SCR can be restored by executing a rich spike as described above to supply a reducing agent to the NSR to cause the NSR to generate NH3.